Unsupported filename characters

[Evil Olverlord]

I've seen several posts on this, but no solution...

ReadyNAS NV+
RAIDiator 4.1.7 [1.00a043]

I use a WD external USB drive to back up the NAS. In fact, I have two, and I alternate which one is on-site. One of the WD drives ("drive E") is formatted EXT3, the other ("drive F") is FAT32.

I used drive F as the backup, and all was fine. I switched to drive E, and all was fine. I switched back to drive F. I deleted all files, and set the backup jobs for full first-time backup.

Three of four jobs ran fine. The final one (media - mostly FLAC) failed due to "Error encountered copying data from source path ==> /WD_Off-site_bck//Media due to unsupported filename characters in the destination filesystem." I reformatted the entire drive (FAT32 again), recreated the backup folders, ran the jobs again. Same result. The log seems to indicate the backup completed all files - it specifies no errors except the text above; no specific problem files noted.

I could reformat drive F to EXT3, but ... it used to work fine.

Thoughts on what's going on?