I rather randomly checked the status of my RAID arrays with cat/proc/mdstat and realized, that one of my arrays seems to be resyncing:md1 : active raid1 sdb7[1] sdc7[0]      238340224 blocks [2/2] [UU]      [==========>..........]  resync = 52.2% (124602368/238340224) finish=75.0min speed=25258K/secWhy is this and what does it mean? I seemingly can access the mount point just fine with r/w permissions.EDIT 1 (in response to SLM's ANSWER)I can't really see anything if I grep through dmesg and the --detail switch doesn't tell me much either, i.e. it displays that the resync is in progress... but no hint for the reason or why it might have gotten out of sync... - I guess I might just need to keep an eye on it before I start swapping out my hardware.
RAID resyncing automatically?
mdadm;synchronization
This would seem to be indicating that the syncing between the 2 members of the RAID are not staying in sync with each other. 1. Investigate logsI'd investigate your dmesg logs and see if there are any messages stating that either of the physical HDDs that make up this array are having hardware failures. 2. Check mdadmYou can also consult mdadm using the --detail switch to find out more information about the resync like so:$ sudo mdadm --detail /dev/md0/dev/md0:        Version : 00.90.03  Creation Time : Sat Jan 26 09:14:11 2008     Raid Level : raid1     Array Size : 976759936 (931.51 GiB 1000.20 GB)  Used Dev Size : 976759936 (931.51 GiB 1000.20 GB)   Raid Devices : 2  Total Devices : 2Preferred Minor : 0    Persistence : Superblock is persistent    Update Time : Fri Jan  1 01:29:16 2010          State : clean, resyncing Active Devices : 2Working Devices : 2 Failed Devices : 0  Spare Devices : 0 Rebuild Status : 50% complete           UUID : 37a3bfcb:41393031:23c133e6:3b879f08         Events : 0.2178969    Number   Major   Minor   RaidDevice State       0       8        1        0      active sync   /dev/sda1       1       8       17        1      active sync   /dev/sdb1If both devices seem fine and you cannot pinpoint which device is having an issue, you may want to temporarily run a diagnostic tool such as HDAT2 or SpinRite against each HDD to confirm their health. 3. CablingIf the HDDs check out then I would start scrutinizing the cabling, I typically will swap these out.4. ControllerI'd next scrutinize the controller itself, either taking the drives out of the affected system and diagnose them in a secondary system, or add a 3rd party controller card into the affected system to diagnose the issue further.5. Power supplyBelieve it or not, I've had issues in the past with HDDs and RAIDs where swapping out a failing, or about to fail, power supply, resolved my RAID health issues.