$ sudo tune2fs -r1 /dev/sda
tune2fs 1.42.12 (29-Aug-2014)
tune2fs: Filesystem has unsupported read-only feature(s) while trying to open /dev/sda
that’s because it can’t act on an ext4 fs because it’s old/lacking compile flags supporting ext4 maybe???
Is probably because that version of tune2fs doesn’t support metadata_csum flags. You could try geting 1.43.3-1 from jessie-backports if that solves your issue.
I didn’t need a work around as I just pulled the drive and changed reserved for root % to 0 on my desktop, it was a suggestion so weirdos like me who have ext4 formatted USB flash drives don’t have to resort to doing that.
Cheers.