@cas You wrote: Is there supposed to be some level of compression built in there? Yes, I think so. See also in OpenWrt's Filesystems About my second 'Trial and Error' example You wrote: The size: 512.0K - That's ok The used: 196.0K - That's not empty So something looks off with that partition. Unfortunately I don't know the jffs2 filesystem too well. (Frankly speaking I don't know it at all I guess the "used 196.0K" on this "empty" mtd7 can be some kind of 'system administration area' - eg. for journalling and so. Please look at the 'Used / Available' figures during the whole process. It's quite funny interesting - isn't it? # cp random.txt (100,000 bytes) five times into/mnt/mtdblock7/ # cut from the output of df -h Filesystem Size Used Available Use% Mounted on /dev/mtdblock7 512.0K 196.0K 316.0K 38% /mnt/mtdblock7 # empty /dev/mtdblock7 512.0K 272.0K 240.0K 53% /mnt/mtdblock7 # after cp the first file /dev/mtdblock7 512.0K 352.0K 160.0K 69% /mnt/mtdblock7 # after cp the second file /dev/mtdblock7 512.0K 432.0K 80.0K 84% /mnt/mtdblock7 # after cp the third file /dev/mtdblock7 512.0K 508.0K 4.0K 99% /mnt/mtdblock7 # after cp the fourth file - almost full cp random.txt /mnt/mtdblock7/random5.txt cp: write error: No space left on device /dev/mtdblock7 512.0K 512.0K 0 100% /mnt/mtdblock7 # after cp the fifth file - full BTW I'm only interested on wear leweling now.