Can you use a Ring of Spell Storing to store a spell cast from a magic item?

Question

Some magic items, like the Staff of the Magi, let you cast spells.

The Ring of Spell Storing lets you store spells for latter use (by yourself, or somebody else).

Can you store spells cast from magic items (such as a Staff of the Magi) in a Ring of Spell Storing?

Answer 1

No

You have to use a spell slot to store a spell in a Ring of Spell Storing. This is because:

The level of the slot used to cast the spell determines how much space it uses

This is backed up by Jeremy Crawford, lead designer, on Twitter:

[Querent:] Can you cast a spell into a Ring of Spell Storing from a wand?

[...]

[Crawford:] It requires a spell slot. A wand doesn't expend a spell slot.

Answer 2

Yes.

• Some magic items allow the user to cast a spell from the item.
• ... and the user must concentrate if the spell requires concentration.

DMG pg. 141 [Spells]

• While holding it, you can use an action to expend 1 or more of its charges to cast the magic missle spell from it.

DMG pg. 211 [Wand of Magic Missles]

Magic Items treat you as if "you cast the spell". All the requisites for prepping the spell are already stored by the original caster.

• You can increase the spell slot level by one for each additional charge you expend.

DMG pg. 211 [Wand of Magic Missles]

In regards to "spell slots" those are stored in the magic items, i.e. wand when created. Many even state "you can increase the spell slot by one for each additional charge you expend."

By RAW, you should be able to cast a spell from a wand in order to store it in a ring of spell storing. The wand has every thing you need to cast the spell, using the item by expending a charge is merely releasing the already stored spell.

Otherwise, you aim to break the mechanics of magic items spells vs spell casters.