To be honest, I think you'll find that 80Gb drives are not much different price-wise to larger drives, since 80Gb is considered smaller than small now

. Ebuyer still have an 80Gb Western Digital PATA 2.5" drive for under £40 - SATA drives of the same type are around the same price. When you look at larger drives you notice the difference more - I just bought a 320Gb 2.5" SATA for my PS3 for £50. Obviously, with the PVR80 you won't really want anything larger than 160Gb as it won't be used and might not work anyway.
I was pondering about getting a 320Gb 3.5" SATA drive for my T816, since SATA->IDE adaptors are only £3 on fleabay. But, having said that, by the time I'd bought other necessary cables it'd be just as cheap to get a PATA drive instead. Also, I'm not sure how big those adaptors are, but since it's a PCB that slots upright into the connector on the motherboard, I'm thinking it might be too high for the Vestel casing.
Of course, the other consideration once you start adding things up is whether buying a T825 is more cost effective, especially when they're on offer. For example, £35-40 for a new HDD (80Gb or 160Gb) or, £66 for a 160Gb T845 or £80 for a 160Gb T825. I didn't realize those T845 Top Up TV boxes were now so cheap, especially compared to the T825 - guess they are not so popular. I think I might have to check with Vestel whether they have any non TUTV software for the T845

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I guess we'll have to take the posters word on that other thread, seeing as I haven't tried it myself. I'm not entirely sure that HDD power in spinup would affect the tuners directly. I get the impression that it's more a lack of cpu "power" - the software is working the cpu too hard and it can't keep up. Certainly this seems to be the case on the T816 and later. Then again, I guess somebody could always test the PVR by powering the drive externally - or did somebody already try that (seem to remember a thread about it).