When you say unplugging it quickly, do you mean cycle the power for the 8300HD box itself? (or the power cable for the hardisk?)
I should mention it's NOT plugged into Videotron if that makes a difference. I had another Explorer STB even refuse to boot up without Videotron's "blessing" via an active cable TV connection.
Does the hardisk need special formatting or partitioning? I formatted it with NTFS under Windows XP, maybe the STB likes FAT32 better?