I would really be doubting the integrity of this iRiver. Sounds like the harddrive is having emphasimic fits, and is not long for this earth.
As far as stealthable 24bit drives go, I don't think that there are very many. You may want to search through the "Recording Gear" section some first, in order to familiarize yourself with what's available for your particular requirements, then ask more detail questions as they arise.
Keep in mind too, that there exists a DIY option to keep that iRiver of yours alive a while longer:
Compact Flash Mod. A decent enough number of people are running this, should be stable (more so that you current drive, anyways, which is undoubtedly going to die sometime, probably soon).
As a closing remark, I just made the leap from h120 to R-09HR and couldn't be happier. Bear in mind though, the R-09HR has no optical / digi in
Edit to add recommended reading:
Best stealthable 24bit device? *
Stealthable 24/96 recordersRecorder market overview: price/feature chart*- this is the thread I started to ask almost all the same questions you should be asking (minus the digi-in question)