I believe that is because once formatted in your NAS, Windows no longer recognises the drives as either unformatted or suitable for use with Windows.
You can get low level formatting tools which would return the drives to a state suitable for use by Windows, BUT……. be very careful in their use as they can easily turn drives into paperweights.
Thank you for the speedy replies, must be getting old (well I am) but not too confident, so may just have to use them as paperweights, will think about it over the weekend.
A couple of old pre-used drives of unknown condition must be worth nearly nothing. So if it were me I would just put them to one side and sometime later get round to physically destroying them.
But it’s not even £80 certain profit. There will be selling costs if using eBay, dealing with any issues where the buyers can’t make them work and handling returns and disputes. They possibly wouldn’t sell anyway. It’s just not worth the bother.
I went thru a similar HDD nearly full a while back.
All my files were WAV & DSD so based on reports from others and my own experiments including using Asset on the fly transcoding I elected to convert WAV to FLAC (5).
Using dBpoweramp Batch Converter it took a few hours including overnight, but it was faultless and well worth it. It saved a load of HDD space, enough to take me to the time when the NAS is past its best and maybe SSD is a lot more sensible.