That's interesting rtguille... I'm trying to recreate that now. I've never personally noticed any "Eject"ing issues after playing an .iso.
But, just in the vein of this whole discussion, I can offer a few other observations.
Let's say you just disconnect a USB drive from a PC or a Mac, without "Safely Remove/Eject"ing it. We've all been told that this can/will corrupt the drive, and in fact many users end up confirming this... they disconnect the drive from their Mac, let's say, and then when they try to re-connect it the Mac won't recognize it because they've corrupted the file system. And yet, apparently, the exact same corrupted drive can be read by *nix... *nix isn't really repairing the corruption without being asked/told to, and merely connecting the corrupted drive to *nix and then back to the Mac doesn't really get the drive working again -- it's still corrupted. It's just that *nix seems better positioned to ignore some file system corruption -- or at least not cry about it.
So, what's this got to do with WDTV's? Well, as you may or may not know, they're *nix based. Ergo, from the experiences I've seen in the PC/Mac world, they are in a position to not yell and scream at you if you connect a corrupted drive -- they don't say anything and just bash away as best they can. But, for all I know, using the drive without fixing the file system errors will just make things worse in the long run. At some point you really do need to repair the file system (whether you know it's borked, or not) or things eventually will start going badly wrong with your data.
It seems as if 1.06.15 is checking for file system errors on boot (or when a drive's attached), from the new behaviours we're seeing, and it looks like it's trying to correct some of them. I can only guess that it seems to be doing this because file system errors were largely ignored in the past, and would presumably become worse and worse, and data was eventually being lost.
That would seem to explain why some drives (no errors) scan very quickly, and why others take 20 minutes or never finish at all... it would seem to depend on just how corrupt the file system has become, as to how much attempted fixing is necessary.
Of course, I am taking a stab in the dark here, although my guesses seem logical to me based on my observations.
It certainly appears that, if you have a drive that's slow to scan, connecting it to a PC and repairing the file system and then "Safely Remov"ing it and hooking it back up to the WDTV does cut the scan time down to a reasonable amount, in line with the time taken by previous firmwares.