ValueError: datasets' indices [1] come from memory and datasets' indices [0] come from disk
Hi ! To help me understand the situation, can you print the values of  `load_from_disk(PATH_DATA_CLS_A)['train']._indices_data_files` and `load_from_disk(PATH_DATA_CLS_B)['train']._indices_data_files` ?
They should both have a path to an arrow file

Also note that from #2025 concatenating datasets will no longer have such restrictions. Sure, thanks for the fast reply!

For dataset A: `[{'filename': 'drive/MyDrive/data_target_task/dataset_a/train/cache-4797266bf4db1eb7.arrow'}]`
For dataset B: `[]`

No clue why for B it returns nothing. `PATH_DATA_CLS_B` is exactly the same in `save_to_disk` and `load_from_disk`... Also I can verify that the folder physically exists under 'drive/MyDrive/data_target_task/dataset_b/' In the next release you'll be able to concatenate any kinds of dataset (either from memory or from disk).

For now I'd suggest you to flatten the indices of the A and B datasets. This will remove the indices mapping and you will be able to concatenate them. You can flatten the indices with
```python
dataset = dataset.flatten_indices()
``` Indeed this works. Not the most elegant solution, but it does the trick. Thanks a lot! 
Hi there,

I am trying to concat two datasets that I've previously saved to disk via `save_to_disk()` like so (note that both are saved as `DataDict`, `PATH_DATA_CLS_*` are `Path`-objects):
```python
concatenate_datasets([load_from_disk(PATH_DATA_CLS_A)['train'], load_from_disk(PATH_DATA_CLS_B)['train']])
```
Yielding the following error:
```python
ValueError: Datasets' indices should ALL come from memory, or should ALL come from disk.
However datasets' indices [1] come from memory and datasets' indices [0] come from disk.
```
Been trying to solve this for quite some time now. Both `DataDict` have been created by reading in a `csv` via `load_dataset` and subsequently processed using the various `datasets` methods (i.e. filter, map, remove col, rename col). Can't figure out tho...

`load_from_disk(PATH_DATA_CLS_A)['train']` yields:
```python
Dataset({
    features: ['labels', 'text'],
    num_rows: 785
})
```
`load_from_disk(PATH_DATA_CLS_B)['train']` yields:
```python
Dataset({
    features: ['labels', 'text'],
    num_rows: 3341
})
```