Hard Disk keeps disappearing during song playback

So, i have a weird problem

i recorded a song made up of six sequences and added HD tracks
everything worked out fine, sounded great

a couple of days later i came back to the song and added a couple of new sequences at the end to extend it out
so, the first weird thing that happened was, half way through playback the HD Tracks just stopped.
rebooted the MPC, played the song again and it was fine
added the new sequences.. same thing
but now i'm getting freeze ups too
and the HD disappears if i'm working on that song

now what's interesting is, when i add the new sequences to the song, the Reps field says Hold... and it didn't do this pre adding the HD Tracks

so my question is this..
are you only supposed to add HD tracks to a finished song?
is the adding of new sequences to a song that already has HD tracks assigned an issue?