Yes, you can deduct depreciation on an asset converted from personal to business use. Until the end of 2022 you're allowed to take 100% bonus depreciation in the first year you put an asset into service, so no need to worry about tracking depreciation year over year, you can take the full deduction immediately.

However, the deduction would be based on the fair market value when you convert it to business use, not what it was worth when you bought it. So you have to figure out what a used, few years old MacBook would sell for, and deduct that, not its original price. Unless it's somehow worth more used than new, in that case only deduct the original cost.

You also have to determine how much you use it for personal vs business purposes and only deduct the appropriate percentage, you can't deduct the whole amount unless you use it 100% for business.

So for example, say you look on ebay and determine that your computer would sell for about $800 used. And say you use it 60% for business use. You can deduct $800 * 60% = $480. Since 100% bonus depreciation is still in effect, you can deduct that whole amount this year (otherwise you'd have to split that deduction over the next 5 years).

Your tax write-off is not a tax credit. If I buy a $1,000 item and it's tax deductible. You won't pay taxed on that $1,000.

So if you make 100k you'd only pay taxes on 99k. Let's say you pay 30% taxes, 30% of 100k is 30k and 30% of 99k would be 29.7k. you'd save $300 in taxes but still spent 1k on the item.