There should be no problem streaming 4k from HDDs as others said. As to SSDs, depends how long are you going to store them without power but I'd say they will be fine for several moths or even more. Again, backups. Also, I'd run CrystalDiskInfo to check drives health: https://www.pcsteps.com/2530-predict-hard-disk-failure-crystal-disk-info/
Data Hoarder
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Max UHD disc speed is 128Mb/s / 16MB/s. Hard drives read at up to ~150-200MB/s, so far from being a limiting factor. Even the slowest 5400 RPM hard drive is still fast enough to play any 4K video.
Even SMR hard drives which do slow down as you full up the drive and/or rewrite files, are fine as the the read speed is the same as CMR drives.
Your issues with crashes and stutters is caused by something else other than the hard drive.
Note that this is for video playback. If you're writing or editing video, then an SSD makes a difference.
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The 8TB Samsung drives are what, $400? Get yourself a 5400RPM drive for half the cost and twice the space.
Are filled 7200 drives still fast enough to play 4K BD rips?
Even the el-cheapo acoustic tuned 14TB 5400RPM Western Digital externals (WD140EDFZ) can maintain sequential speeds in excess of 100MiB/s when full (~ 110-200MiB/s). That's a much higher data rate than BD-ROM drives, so yes. Shouldn't struggle at all unless your files are extremely fragmented and the drive heads are thrashing or you are accessing many files simultaneously.
Thanks really helpful