Running out of disk space, start saving new data to different disk I'm currently at around 7 billion rows on one table and approaching the max disk space for the entire database.

I need to start saving new data on the one large table to a new disk.

I've looked at partitioning by datetime, which seems the obvious choice
This is an interim solution before we get a new server in 6 months, but need a "now" solution.

How would I achieve this? Can I implement manual partitioning so all new data goes to the new disk?

Everything I've seen requires creating something like a monthly option, whereas I just want to put all new stuff on a new disk.

One option is to split the existing data by month, and then manually move the partitions around.

I'm not even sure if this is possible, so would appreciate some feedback on the best solution to achieve the above.

SQL Server 2017 Enterprise Edition.
The normal short-term solution here is to simply move some indexes and tables to the new filegroup to free up space in the primary filegroup.

But


  Can I implement manual partitioning so all new data goes to the new disk?


Yes you can.  The key idea is to create a new table on a partition scheme that is compatible with the existing table, and partitioned on a clustered index key column.  Then switch the table into a partition on that partition scheme, and split its partition function so new rows go to the new filegroup.  

And I just discovered that you can create a partitioned table with only the "primordial" partition, and then switch a non-partitioned table into the primordial partition without needing a CHECK constraint on the non-partitioned table.

Here's an end-to-end demo:

use master 
go
drop database parttest 

go
create database parttest 
go

use parttest 
go
drop table if exists large
go
create table large(id int identity primary key, a char(1000), b int, c datetime);
create index ix_large_b on large(b);

--load some data
with q as 
(
  select top 100000 row_number() over (order by (select null)) n
  from sys.messages m, sys.objects o
)
insert into large(a,b,c) 
select replicate('z',n%989), n%39, getdate()-n%1000
from q;

--add a new filegroup with a single file
alter database current 
  add filegroup newfg

alter database current 
  add file (name = 'newfg_data', filename = 'c:\temp\newfg_data.mdf')
  to filegroup newfg 

--create the new partition function and partition scheme 
--but don't specify any boundary points, so the target
--table can be switched in without a check constraint
create partition function pf_large(int) 
  as range right for values ()

 create partition scheme ps_large 
   as partition pf_large all to ([Primary])

--create the table to switch into
create table large2(id int identity primary key, a char(1000), b int, c datetime)
 on ps_large(id);

--create the secondary indexes on the new table
--if you don't create them they won't be switched
create index ix_large2_b on large2(b) ;

alter table large switch to large2 partition 1

-- drop and rename
begin transaction
  drop table large 
  exec sp_rename 'large2','large'
commit 

--reset the identity values
dbcc checkident('large') 
go

--split the partition function so new rows to to the new filegroup
alter partition scheme ps_large next used newfg 

begin transaction 
  declare @splitPoint int = 1+(select max(id) from large with (tablockx) )
  alter partition function pf_large() split range (@splitPoint)
commit

go

--load some more data 
with q as 
(
  select top 100000 row_number() over (order by (select null)) n
  from sys.messages m, sys.objects o
)
insert into large(a,b,c) 
select replicate('z',n%989), n%39, getdate()-n%1000
from q;

go

--check that it went to the new filegroup
select i.index_id, p.partition_number, ds.name fg, au.data_pages
from sys.tables t
join sys.indexes i 
  on t.object_id =i.object_id
left join sys.partitions p 
  on p.object_id = t.object_id
  and p.index_id = i.index_id
left join sys.allocation_units au
  on (au.type in (1,3) and au.container_id = p.hobt_id)
   or(au.type = 2 and au.container_id = p.partition_id)
left join sys.data_spaces ds 
  on ds.data_space_id  = au.data_space_id
where t.object_id = object_id('large') 
order by ds.data_space_id, index_id, partition_number 


outputs 

index_id    partition_number fg           data_pages
----------- ---------------- ------------ --------------------
1           1                PRIMARY      14286
2           1                PRIMARY      174
1           2                newfg        14286
2           2                newfg        231


You will have to drop and recreate foreign key constraints referencing this table.  And recreating the FK constraints after the switch the switch require table scans during DDL, so can be lengthy offline operations.  To optimize for up time you can recreate the FK constraints with NOCHECK and CHECK them later.  Until then they will be enforced for DML but not trusted by the query optimizer.