I have three different 8MB memory units, each a slightly different size - 623 blocks, 622 blocks, and 609 blocks. The genuine MS one (Part number X08-25319) is 623 blocks.
I use them as follows:
623 block unit: Krayzie 1.1.1 007 Gamesave. 502 blocks used, 121 blocks free
622 block unit: Krayzie 1.1.1 MA Gamesave. 502 blocks used, 120 blocks free
609 block unit: Krayzie 1.1.1 SC Gamesave. 498 blocks used, 111 blocks free
On average that looks like roughly 80% of the blocks used. Now, there's always a difference between block usage (which I guess equates to clusters) and actual file size usage because of the wasted cluster space at the end of each file, but the Krayzie 1.1.1 SC gamesaves are 5957283 + 16490 = 5,973,773 bytes i.e. 75% of the 8MB used, so that roughly ties in with the block usage, and it doesn't matter which measure you use there is around 20% space to spare.
Hence the bottom line is that there seems to be something wrong with your Krayzie gamesaves, or you've got something on there that doesn't need to be there. My guess is you've tried to put the Xtras folder on there as well, so I'd suggest you wipe the MU and start again, with just the unpacked numbered folder (or the two folders for SC) on there.
You might also want to check how many blocks there are in total on the MU as well, because I guess it's possible that some of the blocks are unavailable if the flash chip has started to fail.
This post has been edited by Heimdall: Feb 18 2011, 11:12 PM