Blizzard has updated their minimal necessary spec to add 100GB storage space for TBC Classic Gold both SSDs and hard drives. However, they warn players who performance of your hard drive may influence Shadowland's gaming experience.
A few Shadowlands beta testers have reported that running the cheap WOW Burning Crusade Classic Gold game in an HDD has not influenced their performance or gameplay in any substantial way. However, as this expansion is still in beta, this can be subject to change.